1. Field of the Invention
The present invention relates to capacitance probes and more particularly to capacitance probes utilized in measuring the level of substances which may build up an electrical static charge on the probe.
2. Description of the Prior Art
It has been common practice to measure the level of substances in containers by means of capacitance probes of the type that exhibit a linear change in capacitance corresponding to the level to which the substance rises on a vertically projecting probe. Such probes typically include a central conductor projecting vertically into a vessel to form one plate of a capacitor and connected to one side of a detection circuit, the second conductor or plate of the capacitor being formed by the walls of the container, or by the substance itself and being connected to the opposite side of such detection circuit.
The central conductor is normally encapsulated in an electrical insulator to thus separate such conductor from the substance itself. In the detection of the level of dry substances such as metallic or plastic pellets, the pellets themselves rub together causing a buildup thereon of an electrical static charge. Such static charge often provides a voltage high enough to break down the insulator, resulting in conduction through the probe to the detector circuit itself, resulting in destruction of various components of the detection system.
Previous efforts have led to the incorporation of protective devices such as zenor diodes in the probe circuit itself. However, zenor diodes add an uncompensated capacitance to the probe circuit, thus resulting in uncorrected error in the detection signal. Further, such zenor diodes frequently fail with the discharge of high static voltages, thus exposing the detection equipment to high static voltages and rendering it inoperative. Further, neon lamps have been proposed for protecting the detector circuit but their breakdown voltage is frequently above the threshold level which will still cause damage to the detector circuit.